This invention relates generally to serially sensing the output of multilevel flash memories.
Data is read from a nonvolatile memory by comparing a voltage or current generated from the stored data with a reference voltage or current. In the case of a multilevel cell, the process of reading data involves comparing the voltage or current generated from the data to a series of references in order to determine the position of the generated voltage relative to the references.
Conventionally, the sensing scheme for reading data from multilevel cells involves using one sense amplifier for each reference level. This approach may be called parallel sensing. In contrast, in serial sensing, a single sense amplifier may be utilized to sense the multilevel cell. This has the advantage of reducing the amount of silicon area used for sensing circuitry by a factor of N where N is the number of reference voltages. In addition, the serial sense scheme matches the load on the cell dependent voltage or current with the load on the references. In contrast, in the parallel sense scheme, the cell dependent voltage or current has N sense amplifier loads while each reference has only one sense amplifier load. This mismatch can create offsets in the amplification that may necessitate additional circuitry or may call for greater differential margin.
In view of the advantages of serial sensing, there is a need for better ways to implement serial sensing in multilevel memory cells.